The present invention relates to a method of manufacturing steel powder to be used in the manufacturing of precision components by means of powder metallurgy. Powder manufactured according to the invention makes it possible to simplify the manufacturing of such precision components having great demands for dimensional accuracy as well as mechanical strength.
Powder metallurgical manufacturing is characterized by long series production of components having good dimensional accuracy. The manufacturing sequence is started by mixing iron powder with lubricant in order to simplify the following compression operation wherein the iron powder mixture is compressed to a green which closely corresponds to the desired shape of the final component. The green is thereupon heated to a temperature at which the component is by means of sintering provided with its final features with regard to strength, ductility etc.
It is often desirous to give the component more accurate dimensions than obtained by means of the above procedure. This is provided by means of a further compression operation, a calibration, wherein the component is given extremely accurate dimensions.
In order to provide the components manufactured by means of powder metallurgy with the high strength which is often required there is used alloying powder as the starting material. Nowadays there are used substantially two types of such alloying powders, namely powder mixtures and so called atomized powders.
Powder mixtures are prepared by mixing with the iron powder a powder of the alloying element, either in the elementary form or as a compound which is decomposable during the sintering process. The so called atomized steel powders are manufactured by comminuting a steel melt containing the desired alloying elements to a powder.
One of the drawbacks of powder mixtures is the risk of segregation which risk is present because of the fact that powders having different characteristics, for example different particle sizes, are mixed with each other without being mechanically connected. This segregation leads to varying composition of the green compacts manufactured from the powder mixtures which in turn leads to varying dimensional changes during the sintering thereof. Another drawback of powder mixtures is their tendency to dust especially when the alloying element is present in the form of very small particles. Of course, this fact can lead to difficult environment problems.
On the other hand the atomized powder completely lacks the risk of segregation as every powder particle has the desired alloying composition. Also the dust risk is reduced as no alloying element having small particle size is included. On the contrary the pre-alloyed atomized powder has another great drawback, namely the low compressibility thereof which is dependent on the dissolvent annealing effect which the alloying elements have on each powder particles. A high compressibility is essential when there is required a component having high density which is a prerequisite for high strength. On the other hand the compressibility of a powder mixture is substantially the same as the compressibility of the iron powder included therein. This fact together with the flexibility with regard to the alloying composition which characterizes powder mixtures have made powder mixtures to the most commonly used form of alloying powder.